STUDIES IN SPICULE FORMATION. 141 
spicules, which are adult, or nearly adult, in Trichasterina 
borealis. 
Notwithstanding the large quantity of material which has 
been placed at my disposal and my thorough examination of 
this material, I have been very little more successful than the 
two investigators just named in discovering the early stages 
in the development of hexactinellid spicules.! he earliest 
stage which I have found is indeed somewhat younger than 
that figured by Ijima, but it throws very little, if any, extra 
light on the question as to the condition of the scleroplasm 
at the time of origin of the spicule and as to the early 
morphogenesis of the spicule itself. The young stages 
which I have figured were present in Rossella podagrosa, 
Kpk., the histology of which species was comparatively well 
preserved. I have only figured the scleroplasmic investment 
of spicules from this and the allied species, R. antarctica, 
because in the other genera which I have studied the spicules 
were 1n precisely the same condition. 
Tjima’s remarks on the scleroplasm in connection with the 
young spicules described by him are as follows:—“A 
hexaster begins its development as a hexactin. ... The 
hexradiate principals, during the entire period of development 
of both the floricome and the graphiocome, are imbedded in 
a body of protoplasmic substance, enclosing a crowded 
number of nuclei. This nucleated substance may not im- 
properly be called the scleroblast-mass. . . . At first, so long 
as the terminals are yet undeveloped or are very short, the 
mass may be said to present a more or less octahedral shape, 
with somewhat concave surfaces and with rounded corners. 
In it the three axes of the principals are disposed similarly to 
the axes in a crystal octahedron, the outer ends of the prin- 
cipals coming up very close, but I think normally not quite, 
to the surface at the six rounded corners. The mass may 
otherwise be described as having its surface raised into six, 
radially directed, hump-like protuberances by the six prin- 
1 Tt is worth remarking that the buds of my hexactinellids contained fewer 
young spicules than the older portions of the sponge body. 
